1. Field of the Invention
The present invention relates to unbalanced voltage compensation, and more particularly, it relates to a method and a compensator for compensating for unbalance of three-phase AC (alternating current), and a control method and a controller for compensating for the unbalance of three-phase AC in a three-phase converter which converts three-phase AC power into DC (direct current) power.
2. Description of the Related Art
It is known that occurrence of voltage reduction on the power source side, such as instantaneous voltage drop (voltage sag) and a long-term voltage reduction, has an impact on the load side which receives power supply from the power source, such as production line stop and defects in manufactured goods. In particular, since it has a significant impact on semiconductor manufacturing equipment, standards are established regarding the voltage sag; “SEMI F47-0200” (non-patent document 1), and “SEMI F47-0706” (non-patent document 2). As for a testing method, it is described in “SEMI F42-0600”.
Conventionally, this type of voltage sag has been addressed, for example, by installing an electric storage device such as a voltage sag compensator and an uninterruptible power supply (UPS), which employ a capacitor and a storage battery. The compensator using the electric storage device may have a configuration to be installed as a parallel device either on the power source side or on the load side. Alternatively, it may be installed as a serial device, being inserted between the power source side and the load side, with a configuration that the power system is switched at the time of voltage sag.
It is also known that when a short interruption or instantaneous voltage drop occurs in the three-phase AC input voltages of the (AC-DC) power converter for converting the three-phase AC power into DC power, the power supply to the load is maintained by the voltage sag compensator (for example, see patent document 1 and patent document 2).
FIG. 16 illustrates a configuration example of a conventional voltage fluctuation compensator 102. FIG. 16 illustrates a three-phase AC power source 101 in the form of wye connection having AC power sources 101a, 101b, and 101c for respective phases. However, in the conventional system using the electric storage device, it does not matter which connection is employed, the wye connection or delta connection. The voltage fluctuation compensator 102 is installed between the three-phase AC power source 101 and a DC load (not illustrated). It is to be noted here that the three-phase AC power source with the electric storage device is applicable to any of the wye connection and the delta connection.
In the voltage fluctuation compensator 102, as to the phases for the three-phase AC (a-phase, b-phase, and c-phase), the voltage compensation circuits 104a, 104b, and 104c for respective phases are serially connected, those voltage compensation circuits respectively being provided with capacitors 105a, 105b, and 105c as energy storage means, and a control circuit 103 is provided for controlling the voltage compensation circuits 104a, 104b, and 104c for the respective phases.
The three-phase AC power source is connected not only to three-phase equivalent loads, but also to various single-phase loads. Application of such various loads or influences such as weather phenomenon and accident phenomenon may cause a voltage sag in the state where the three phases are balanced or unbalanced.
The voltage compensation circuits 104a, 104b, and 104c for the respective phases, output compensating voltages for the respective phases based on a command from the control circuit 103, and compensate for the voltage fluctuations. The voltage fluctuation compensator 102 recharges the capacitors 105a, 105b, and 105c during the normal operation. In the event of short interruption or instantaneous voltage drop, the electric power discharged from the capacitors 105a, 105b, and 105c keeps a constant output voltage, thereby continuing power supply to the DC load.
There are following problems in the voltage sag compensator as described above, i.e., not only this system requires equipment investment for installing a large-sized electric storage device or a capacitor unit, but also periodic maintenance is necessary. Therefore, it has been demanded that power is supplied stably by means of converting the three-phase AC input power, even at the time of instantaneous voltage drop, without using the electric storage device. There has been also a demand for a power factor improvement without using the electric storage device.
In order to address the problems above, followings are suggested; considering that in the event of the instantaneous voltage drop, the input voltages during the voltage sag become unbalanced in the three phases, a three-phase PWM converter is employed to compensate for the voltage sag, by controlling the three-phase unbalanced input voltages during the voltage sag period, so as to achieve the voltage sag compensation without using the voltage fluctuation compensator employing the storage device. According to the voltage sag compensation by using the three-phase PWM converter, the electric power having been supplied in the normal state before the voltage sag state can be supplied continuously, even after the occurrence of the voltage sag.
Hereinafter, an explanation will be made how the three-phase PWM converter controls the three-phase unbalanced input voltages.
FIG. 17 illustrates an equivalent circuit in the event of voltage sag. In FIG. 17, er, es, and et represent transmission line voltages balanced in three phases, Z1 represents a transmission line impedance, Z12, Z23, and Z31 are equivalent impedances at the time of voltage sag, eab, ebc, and eca represent line voltages unbalanced in three phases generated at the time of voltage sag, e1o represents a zero-phase-sequence voltage, and Za, Zb, and Zc represent load impedance, which is expressed in the form of load impedance obtained by converting the DC load Rdc (shown in FIG. 18) to the three-phase AC input side.
When the amplitude is assumed as Em, the transmission line voltages balanced in three phases er, es, and et are respectively expressed by the following formulas (1) to (3):er=Em cos ωt  (1)es=Em cos(ωt−2π/3)  (2)et=Em cos(ωt+2π/3)  (3)
Since er, es, and et represent the transmission line voltages balanced in three phases, negative-phase-sequence component en(rst) and zero-phase-sequence component eo(rst) do not appear. Therefore, the negative-phase-sequence component en(rst) and the zero-phase-sequence component eo(rst) are expressed by the following formula (4):en(rst)=eo(rst)=0
In the example of FIG. 17, the state where the voltage sag is occurring corresponds to the state that the equivalent impedances Z12, Z23, and Z31 are applied on the transmission line impedance Z1. On this occasion, the line voltages eab, ebc, and eca go into the three-phase unbalanced state, and cause the zero-phase-sequence voltage e10 as shown in FIG. 17.
As shown in FIG. 18, with respect to the terminals a, b, and c in FIG. 17, the left side illustrates the three-phase AC power source 100B, and the right side illustrates a main circuit unit of the three-phase converter 200. The three-phase AC power source 100B is illustrated in such a manner as being equivalent to the three-phase balanced voltages er, es, and et, and unbalanced factors. In this illustration, application of the impedances Z12, Z23, and Z31 as shown in FIG. 17 expresses the unbalanced factors in equivalent manner.
An unbalanced voltage compensator 400 uses given or measurable three-phase unbalanced input phase voltages to generate compensation signal. The three-phase PWM converter 200 is provided with a three-phase PWM circuit 200a and a three-phase PWM control pulse generator 200b. The three-phase PWM control pulse generator 200b generates control pulse signals based on the three-phase unbalanced input voltages generated in the unbalanced voltage compensator 400, thereby exercising the PWM control over the three-phase PWM circuit 200a. According to the PWM control, the three-phase PWM converter 200 supplies to the DC load 300, the DC voltage to which the unbalanced voltage compensation has been performed.
As described above, the three-phase unbalanced input voltages during the voltage sag are controlled by the three-phase PWM converter, and the compensation for the voltage sag can be achieved without using the voltage fluctuation compensator employing the electric storage device such as a capacitor or a storage battery.
However, in general, in order to control the three-phase PWM converter in which PFC (Power Factor Correction) is incorporated, it is necessary to derive wye-connection three-phase unbalanced phase voltages which are 120° out of phase with each other. After the process for converting derived detection signals into variables on a rotating coordinate system (dq-axis), they are separated into a positive-phase-sequence voltage, a negative-phase-sequence voltage, and a zero-phase-sequence voltage, and they are used as feedback signals which are necessary for the control.
For example, the following non-patent documents 3 to 5 are known as describing the voltage sag compensation according to the three-phase PWM converter control.
In the three-phase PWM converter control described in those non-patent documents, three-phase unbalanced voltages of wye-connection, which are 120° out of phase with each other, are assumed as given or measurable input phase voltages.
On the other hand, a general three-phase power distribution system employs delta connection. In the three-phase power distribution with delta connection, the three-phase voltage which can be actually measured is a line voltage between the terminals, and the wye-connection voltage and the zero-phase-sequence voltage are unmeasurable.
Therefore, in order to compensate for the three-phase unbalanced voltages in the three-phase power distribution being the delta connection, by the three-phase PWM converter control which has been conventionally suggested, it is necessary to derive from the line voltages being measured, three-phase unbalanced phase voltages being the wye-connection, which are 120° out of phase with each other.
As described above, with the control of the three-phase unbalanced input voltages during the voltage sag by using the three-phase PWM converter, it is possible to achieve the voltage sag compensation without using the voltage fluctuation compensator which employs an electric storage device such as a capacitor or a storage battery.
However, in general, in order to control the three-phase PWM converter which incorporates the PFC (Power Factor Correction), it is necessary to derive the three-phase unbalanced phase voltages being wye-connection, which are 120° out of phase with each other. Detection signals being derived are subjected to conversion process to be converted into variables on the rotating coordinate system (dq-axis), and thereafter, these signals are separated into a positive-phase-sequence voltage, negative-phase-sequence voltage, and zero-phase-sequence voltage, so as to be used as feedback signals which are required for the control.
The non-patent documents 3 to 5 are known as disclosing the voltage-sag compensation by the three-phase PWM converter control, by way of example. However, the three-phase PWM converter control described in these documents assumes that the three-phase unbalanced voltages being wye-connection which are 120° out of phase with each other are given or measurable input phase voltages.
On the other hand, a general three-phase power distribution system employs the delta connection. A voltage that can be measured is a delta-connection three-phase voltage, and it is a line voltage between each of the terminals of the delta connection. Therefore, a wye-connection voltage and a zero-phase-sequence voltage are actually unmeasurable. Consequently, in order to compensate for the three-phase unbalanced voltages in the delta-connection three-phase power distribution, by the three-phase PWM converter control conventionally suggested, it is necessary to derive wye-connection three-phase unbalanced phase voltages which are 120° out of phase with each other, from the line voltages being measured. When the voltage sag compensation is controlled by the three-phase PWM converter, it is necessary to convert delta-type voltages of the three-phase unbalanced voltages being received, into wye-type voltages, so as to obtain control parameters. In particular, it is significant to extract a zero-phase-sequence voltage.
For example, patent document 3 is known as disclosing an apparatus or a method for compensating for the instantaneous voltage drop by the control according to this three-phase PWM converter. The instantaneous voltage drop compensator as described in the patent document 3 is provided with a line phase voltage conversion means. The line phase voltage conversion means converts a line voltage signal being detected by a line voltage detection means into a phase voltage conversion signal, and generates a zero-phase-sequence voltage signal and a phase voltage signal from this phase voltage conversion signal.
The line phase voltage conversion means detects peak values of the phase voltage conversion signals (vr′, vs′, vt′), calculates coefficients k1, k2, and k3 based on these three peak values, and generates the zero-phase-sequence voltage signal v0 (=k1·vr′+k2·vs′+k3·vt′) and the phase voltage signals (vr, vs, vt) based on the coefficients k1, k2, and k3 being calculated.    Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-274559 (FIG. 1, paragraph [0018])    Patent document 2: Japanese Unexamined Patent Application Publication No. 2004-222447    Patent document 3: Japanese Unexamined Patent Application Publication No. 2008-141887 (paragraph [0043], from [0055] to [0059])Non-Patent Document 1:    SEMI “SEMI F47-0200 Specification for Semiconductor Processing Equipment Voltage Sag Immunity”, pp. 859-864, issued in September, 1999, as the first edition, and issued in February, 2000 (SEMI 1999, 2000) (SEMI 1999, 2001)Non-Patent Document 2:    SEMI “SEMI F47-0706 Specification for Semiconductor Processing Equipment Voltage Sag Immunity”, pp. 1-12, issued in September, 1999, as the first edition, and approved to be issued in May, 2006 (SEMI 1999, 2006)    Non-patent document 3: J. K. Kang, S. K. Sul, “Control of Unbalanced Voltage PWM Converter Using Instantaneous Ripple Power Feedback”, Power Electronic Specialist Conference, PESC 97, PP. 503-508 (1997-5)    Non-patent document 4: H. S. Kim, H. S. Mok, G. H. Choe, D. S. Hyun, S. Y. Choe, “Design of Current Controller for 3-phase PWM Converter with Unbalanced Input Voltage”, Power Electronics Specialist Conference, PESC 98, pp. 503-509 (1988-8)    Non-patent document 5: S. C. Ahn, D. S. Hyun, “New Control Scheme of Three-Phase PWM AC/DC Converter Without Phase Angle Detection Under the Unbalanced Input Voltage Conditions”, IEEE Transaction on Power electronics, pp. 616-622 (2009-9)
According to the patent document 3, the line phase voltage conversion means converts the line voltage signals being measured, into phase voltage conversion signals, and generates a zero-phase-sequence voltage signal and phase voltage signals based on the phase voltage conversion signals. Accordingly, it is possible to control the three-phase PWM converter based on the three-phase unbalanced voltages (line voltages) of the general three-phase power distribution system, and the three-phase unbalance compensation can be performed.
However, this line phase voltage conversion means detects peak values of the phase voltage conversion signals obtained by converting the line voltages, and generates the zero-phase-sequence voltage signal and the phase voltage signals according to the coefficients calculated based on these three peak values. Therefore, in order to generate the zero-phase-sequence voltage signal and the phase voltage signals, it is necessary to repeat measuring the line voltages more than once, so as to obtain the coefficients being optimum, and there is a possibility that a longer time may be taken to generate the signals.
If unbalanced voltages and phase angles of the wye-phase voltages are given, the line voltages being unbalanced can be determined from those wye-phase voltages according to a standardized manner. On the other hand, even though the unbalanced voltages and the phase angles of the line voltages are known, the wye-phase voltages cannot be determined according to a standardized manner from these line voltages. This is because a reference point of the wye-phase voltages cannot be specified, and there is infinite number of combinations of wye-phase voltages having the same unbalanced voltages and phase angles.
In order to control the three-phase PWM converter, it is necessary to have a relationship of 120° out of phase between the wye-phase voltages. Therefore, specific wye-phase voltages which are 120° out of phase with each other have to be selected from the infinite number of combinations of wye-phase voltages. When these specific wye-phase voltages which are 120° out of phase with each other are selected, a positive-phase-sequence voltage becomes in phase with a particular phase (a-phase) of the wye-phase voltages, and a DC component as a control target can be extracted according to the subsequent dq-axis conversion process. Therefore, it is convenient for the control of the three-phase PWM converter. In addition, a phase angle of the negative-phase-sequence voltage and a phase angle of the zero-phase-sequence voltage with respect to the positive-phase-sequence voltage, indicate the same angle in the direction opposite to each other, therefore enabling the zero-phase-sequence voltage to be derived.
Conventionally, in order to obtain from the line voltages, the wye-phase voltages which are 120° out of phase with each other, it is necessary to detect the unbalanced state of voltages and further to select the wye-phase voltages which are 120° out of phase with each other, from the line voltages being measured. Therefore, it may take a longer processing time. By way of example, when the unbalanced state of voltages is detected in alternating current, it is necessary to monitor voltage fluctuations during at least ½ cycle.
In order to quickly compensate for the voltage unbalance by controlling the three-phase PWM converter, it is requested to reduce the time required for detecting the unbalance in voltages and generating a control signal, and thus it is necessary to derive instantaneous wye-phase voltages from instantaneous line voltages. It is to be noted here that the instantaneous line voltages are line voltages measured at a certain point of time, and the instantaneous wye-phase voltages are values of the line voltages derived based on the actual measured values of the line voltages being obtained at this point of time. These wye-phase values correspond one-to-one with one point for measuring the line voltages, and this means that the wye-phase voltages can be obtained from values measured at one measuring point of time, without requiring measured values at multiple points.
In order to quickly eliminate an influence caused by the instantaneous voltage drop on the load side, it is necessary to immediately generate the zero-phase-sequence voltage signal and the phase voltage signals which are required for controlling the three-phase PWM converter, in response to the fluctuation of the unbalanced state of the three-phase line voltages in the three-phase power distribution system. In the line phase voltage conversion means described above, it is expected that actual measurement of the line voltages is repeated more than once in order to generate the zero-phase-sequence voltage signal and the phase voltage signals. Therefore, there is a possibility that the response to the fluctuations in the unbalanced state of the three-phase line voltages may be insufficient.
As a response to the instantaneous voltage drop, the specification for voltage sag immunity SEMI F47-0200 is known, for instance. The specification for voltage sag immunity SEMI F47-0200 defines a range in a wide band (a range of input voltage reduction from 0% to 100%) to be controlled by the voltage sag compensation. In this voltage sag immunity specification, it is defined that the voltage reduction rate within 0.2 seconds from the occurrence of voltage sag is down to 50%, and the voltage reduction rate from 0.2 seconds to 0.5 seconds is down to 70%, and the like.
If the response to the instantaneous voltage drop is insufficient, it is difficult to satisfy this specification voltage sag immunity.
Conventionally, the following technique has not been known; i.e., using the three-phase unbalanced voltages of line voltages as the input voltage, instantaneous wye-phase voltages are derived from the instantaneous line voltages, thereby controlling the three-phase PWM converter and compensating for the unbalanced voltages.
An object of the present invention is to solve the conventional problems, and to derive instantaneous values of wye-phase voltages of the wye-connection which are 120° out of phase with each other, from the instantaneous values of the line voltages, in order to compensate for the unbalanced voltages of three-phase AC. That is, the instantaneous wye-phase voltages are derived from the instantaneous line voltages, thereby controlling the three-phase PWM converter to compensate for the unbalanced voltages.
More specifically, an object of the present invention is to derive a positive-phase-sequence voltage, a negative-phase-sequence voltage, and a zero-phase-sequence voltage, which are three-phase unbalanced voltages of wye-connection being 120° out of phase with each other at the time of actual measurement, from actual measured values of the line voltages, at one actual measurement point, being the three-phase unbalanced voltages which are generated in delta connection. That is, the object is to the three-phase unbalanced voltages of wye-connection which are 120° out of phase with each other, from actual measured values of the line voltages at one actual measurement point, thereby controlling the three-phase PWM converter to compensate for the unbalanced voltages.
Here, the instantaneous values of line voltages are values of the line voltages actually measured at a certain point of time, and the instantaneous values of wye-phase voltages are values of the wye-phase voltages being derived based on the actual measured values of the line voltages.